The invention relates to the general field of integrated circuits with particular reference to light patternable dyes and maintenance of the associated stepper system.
There are two principal types of color resistxe2x80x94dye-based and pigment-based, these pigments being usually red, blue, or green. Most of the pigment types give negative images (when used in photolithography), making them popular for commercial production. In a pigment-based resist, the pigment is about 25% of the total material, the rest being the polymer substrate, various additives and solvent. In practice, it is difficult to dissolve the pigment in the presence of the additives needed to bring about light sensitivity so a substantial amount of solvent is necessary to ensure full dissolution of the pigment.
As a consequence of this, the viscosity of pigment-based resists tends to be low and, furthermore, a certain amount of organic vapor from the solvent gets released during exposure. This brings about contamination of the projection lens surface as well as other parts of the system, which leads to abnormal (i.e. distorted) printed patterns, scum formation, and residues on bond pads. This accumulation of solvent vapor on the lens surface gets worse with time.
It is very important to detect this abnormal situation in time to be able to take corrective action, such as cleaning the lens or replacing contaminated parts. Otherwise, this negative effect seriously impacts the product quality of the color filter and increases production cost due to the rework actions that must be implemented. In Japan, some fabs use an extra projection lens of quartz material to catch the contamination, but this is still not enough for optimum production line operation. If a suitable monitor were available, the production line could become even more economical and competitive.
A routine search of the prior art was performed but no references that teach the exact processes and structures of the present invention were discovered. Several references of interest were, however, encountered along the way. For example, in U.S. Pat. No. 5,998,071, King et al. show a monitor that uses critical dimension bars to detect lens heating effects. In U.S. Pat. No. 5,300,786, Brunner et al. show a PSM test and monitor system while in U.S. Pat. No. 5,902,703, Leroux et al. show a modified box in box overlay monitor for detecting and measuring dimensional anomalies in photolithographic images formed with a stepper lens.
Bae (U.S. Pat. No. 5,821,131) shows a process of measuring the characteristics for the stepper lens while Ehrlich (U.S. Pat. No. 5,310,624) shows a dry photo process to avoid vapors from photoresist.
It has been an object of the present invention to provide a method for monitoring the lens in a stepper system that is exposed to organic vapors in order to detect changes in optical quality of the system.
Another object of the invention has been to provide a test pattern to be used in conjunction with said monitoring method.
A further object has been to detect these quality changes before product made using the lens is lost.
These objects have been achieved by means of a test matrix (in which exposure time and focal distance are systematically varied) through which a series of images are formed in a pigment-based resist and then evaluated for quality. A key feature is the test patterns that are used for forming the images. Said patterns are designed so as to maximize the total amount of diffraction that occurs during image formation, thereby maximizing sensitivity to changes in lens quality. Several examples of said test patterns are provided.